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STRANDED Mapping the Trends of New England's 2018 Unusual Mortality Event
Elevated numbers of harbour and gray seal mortalities have
been observed along the coasts of Massachusetts, New
Hampshire and Maine. The National Oceanic and Atmospheric
Administration (NOAA) declared this unexpected mass die off
an Unusual Mortality Event (UME) in August, 2018.
The sheer number of deaths in the outbreak and stranding
range appears to be supporting the assumption that the cause
of this event is rooted within infectious disease.
Anjali Narasimhan MCM 591 — GIS for Conservation Medicine
Presented on December 14th 2018
Sincere thanks to Carolyn Talmadge and Miren Schleicher for their unwavering
support and guidance with this project, and to Wendy Puryear for providing the data
set to complete this assessment.
Harbour seals are resident
species of the New England
region.
The data gathered from the
UME had 91 stranded
harbour seals. Kernel Density
was run on the points to
determine the areas of high
and low stranding density
along the coast. Harbour
seals displayed a high density
along the coast of Maine. The
points were characterized by
sex (male = 35, female = 42).
Females strandings were
slightly more prevalent than
males in the UME.
Map: North America featuring the coast of New England.
Characterizing the trends of the UME may provide further
insight into its cause, contributing parameters, and the
magnitude of the event.
GIS and statistical analysis were used to interpret the data of
125 stranded seals collected by marine mammal stranding
response agencies, provided through the Runstadler lab at
Tufts University. The following factors were investigated:
species, sex, age, change in stranding average, and disease.
METHODS
This methodology can be repeated globally to map the
trends in similar mass mortality scenarios involving marine
mammal species.
The information provided by this analysis plays an influential
role in informing marine mammal agencies on the degree of
stranding and prevalence of disease in pinniped populations
of the New England region. This may aid in the future
identification of key areas of high stranding density, to
facilitate marine mammal response units in the event a UME
were to re-occur.
A chart was computed to determine
whether a trend between age class and
stranding existed in the UME data (n=125).
There was evident variation between the
three pinniped species, with the most
frequent strandings occurring in harbor
seal weanlings (n=29). The bulk of the
strandings were in seals ≤ yearlings
(n=106), however it is clear that the UME
epidemic has affected all ages.
RESULTS & DISCUSSION The computed maps supported NOAAs consensus that this
mass mortality event is attributed to infectious disease,
including both influenza and phocine distemper virus. In
addition, this assessment traced the magnitude of this event
along with the potential contributing factors.
Chart displays PDV
prevalence separated
by species. Harbour
and gray seals
exhibited a high rate
of PDV positive
results (n=40).
Phocine Distemper Virus
Influenza Virus Hotspot
When compared to phocine distemper,
influenza virus accounted for fewer positive
stranded seals in the UME (n=16).
Hot spot analysis was run on all the points
testing for IAV to compute significant >90%
hotspots (n=7).
Phocine Distemper Hotspot
Phocine distemper virus accounted for the
majority of the stranded seals that tested positive
for disease in the UME (n= 40).
Hotspot Analysis was run on all the points, testing
for PDV to compute significant >90% hotspots
(n= 17).
The following tools were run to explore the trends of the UME:
Kernel Density - Calculated a magnitude per unit area from
data points (species, sex, disease) to fit a smooth surface.
Raster Calculator - Performed a calculator function using
Python syntax to compute a single surface from the stranding
data of multiple years (2017 and 2018).
Hotspot Analysis - Identified statistically significant hot and
cold spots from disease (phocine distemper and influenza virus).
Stranding has drastically
increased from 2017 to 2018
throughout the coasts of
New England. The current
stranding numbers exceed
those of 2017 by over 10
times. The Kernel Density
was run for the points from
2017 (n=13) and from 2018
(n=125). To characterize the
change, Raster Calculator
was used to subtract 2017
from 2018. There was an
evident decrease in
stranding density along the
coast of MA between Boston
and Cape Cod, and an
exponential increase along
the coasts of ME.
Gray seals are resident
species of the New England
region.
The data gathered from the
UME had 29 stranded gray
seals. Kernel Density was run
on the points to determine
the areas of high and low
stranding density along the
coast. Gray seals displayed a
high density along Cape Cod.
The points were characterized
by sex (male = 3, female = 6).
There was an evident lack of
data regarding sex, so no
significant relationship could
be made.
Harp seals are resident to the
Arctic region. This pinniped is
unusual around the New
England region, but
occasionally migrates to the
Cape.
The data gathered from the
UME had 3 stranded harp
seals. Kernel Density was run
on the points to determine
the areas of high and low
stranding density along the
coast. The points were
characterized by sex (male =
2, female= 1). As there were
very few stranded harp seals,
neither the density nor sex
was significant.
Influenza Virus
Pinnipeds have a global history of succumbing to
disease outbreaks. The increase in stranding has a
direct correlation with the prevalence of infectious
diseases, specifically phocine distemper virus (PDV)
and influenza virus (IAV).
Stranded seals were tested for the presence of both
viruses and Kernel Density was run to determine the
density of disease
in the UME.
Projection: NAD 1983 UTM Zone 19N
Data Sources: Runstadler lab at Tufts University, IUCN Red List, NOAA, GADM & ESRI.
State Number of
Strandings
Maine 1034
New
Hampshire 138
Massachusetts 196
Total 1368
Table: Seal strandings by state during the
UME from July to November 2018.
Harbour Seal Phoca vitulina
Gray Seal Halichoerus grypus
Harp Seal Pagophilus groenlandicus
BACKGROUND
Figure 2: PDV
distribution
Figure 1: Age class of strandings
Change in
Stranding
Density from
2017 to 2018
Disease Density
Harbour seal Range
Gray seal Range
Harp seal Range
Atlantic
Ocean
Atlantic
Ocean
Atlantic
Ocean
Atlantic
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Atlantic
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Atlantic
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Atlantic
Ocean